1. Field of the Invention
This invention relates to the field of protective and/or surface treatment of articles, particularly metal surfaces and to surface treatment compositions employed as solutions or dispersions and methods of using these compositions. The compositions coated on the metal surfaces provide improved corrosion resistance and adhesion of any subsequent overlaying coatings.
2. Statement of Related Art
In the art of treating metal surfaces, it is common practice to improve the corrosion resistance characteristics and bonding of subsequent coatings to the metal surface by depositing or coating the metal surface with a protective coating or film. In order to improve the qualities of the already applied protective coating, it is common to provide a second or subsequent coating after the initial protective coating has been formed on the metal surface. This second coating further enhances the corrosion resistance of the unpainted metal surface and to prepare the surface for reception of a final finish coating where one is to be employed, such as a paint, enamel or lacquer. In U.S. Pat. No. 3,912,548, assigned to Amchem Products, Inc., there is described an aqueous zirconium compound-polyacrylic acid coating composition which is applied to a metal surface already having deposited thereon a conversion coating, which results from treatment of a base metal surface with aqueous solutions which react with the metal surface. The conversion coatings described in the patent are chromate or phosphate coating.
Conversion coatings which include hexavalent chromium have been widely used in the past to protect metals such as iron, zinc, magnesium and aluminum or alloys of aluminum including those with manganese, copper, zinc, silicon and magnesium. While such chromium coatings provide good corrosion resistance properties, recent attempts have been made to produce acceptable chromate-free coatings because of growing concern regarding pollution effects of the chromate and phosphate, particularly hexavalent chromium, discharged into rivers and waterways, which results in extensive waste treatment procedures being required to control their discharge. In U.S. Pat. No. 3,964,936, assigned to Amchem Products, Inc., there is reported and described various attempts to develop chromium-free aluminum coating solutions and describes a zirconium coating solution, which may also include boric acid. In describing coating solutions Examples 29 and 30 in the patent, which include sodium nitrate and cobalt nitrate, Co (NO.sub.3).sub.2 .multidot.6H.sub.2 O, reference is made also to U.S. Pat. No. 3,682,713, which includes a sodium nitrite component in a coating solution for steel and galvanized steel as well as aluminum.
U.S. Pat. Nos. 4,963,596 and 4,970,264 assigned to Henkel Corporation, describe the use of modified polyphenol compounds, which may be used in the "post-treatment" of conversion coated metal surfaces.